1. Field of the Invention
The present invention relates to a plasma type exhaust gas cleaning apparatus in which voltage is applied across a discharge electrode and a ground electrode to create a corona discharge field and the exhaust gas emitted from combustion equipment, such as an engine, is passed through between these electrodes for cleanup.
2. Description of the Related Art
A plasma type exhaust gas cleaning apparatus of this kind is disclosed, for example, in Japanese Patent Laid-Open Publication No.Hei 5-59934. In this plasma type exhaust gas cleaning apparatus, a ground electrode is arranged cylindrically around a linear discharge electrode to constitute a corona discharge tube. The corona discharge tube is arranged on the exhaust path from an engine. Under engine operation, this plasma type exhaust gas cleaning apparatus passes the exhaust gas from the engine through the interior of the cylindrical ground electrode while applying a predetermined voltage from a high voltage generator to between the electrodes. This creates a corona discharge field to produce plasma, which cleans up the exhaust gas. Besides, this plasma type exhaust gas cleaning apparatus has a number of dielectric grains or pellets which are filled into between the electrodes so that electric discharges occur at short ranges between adjoining dielectrics for the sake of uniformity in electric field.
By the way, because such a plasma type exhaust gas cleaning apparatus had the dielectrics of grain or pellet form, the discharge electrode and the ground electrode were in communication with each other through slight gaps between dielectrics. In this case, the plasma from corona discharges threaded through the dielectrics to extend chiefly between the electrodes, hardly occurring between dielectrics. Accordingly, the plasma type exhaust gas cleaning apparatus could not provide a field of adequate uniformity. Thus, in the areas of lower plasma densities such as the vicinity of the outer ground electrode, exhaust gas could simply pass through as uncleaned and be emitted into the air.
An object of the present invention is to provide a plasma type exhaust gas cleaning apparatus which can uniformize plasma density so that a sufficient exhaust-gas cleaning capability is exerted.
This object has been achieved by the provision of a plasma type exhaust gas cleaning apparatus according to the present invention. The apparatus comprises a dielectric arranged in an exhaust path, between a discharge electrode and a ground electrode. The dielectric has a plurality of independent cavities for allowing exhaust gas to flow through.
When each cavity in the dielectric is thus formed independently, the discharge electrode and the ground electrode are partitioned with reliability so that the corona-discharge plasma occurs in each individual cavity without arising directly across the-discharge electrode and the ground electrode. This results in shorter discharge ranges, obtaining plasma of uniform density between the electrodes.
Thus, the exhaust-gas cleaning capability can be exerted sufficiently.
The plurality of independent cavities are arranged along the direction across the electrodes. Then, the expression
Vxe2x89xa7Eaxc3x97(Dsxc3x97xcex5a/xcex5s+Da)xc3x97R 
is satisfied, where V is an impressed voltage required for a discharge between the discharge electrode and the ground electrode, Ea is an electric field required for ionization (decomposition of exhaust gas) acting on each of the cavities, xcex5a is the permittivity of each of the cavities, xcex5s is the permittivity of the dielectric, Da is the total thickness of the cavities in the direction across the electrodes, Ds is the total thickness of the dielectric in the direction across the electrodes, and R is a relative gas density correlating to temperature and pressure.
Accordingly, the total thicknesses Da and Ds of the cavities and the dielectric, the impressed voltage V across the electrodes, and others are put into an ideal relationship.
Thus, detriments such as unnecessary power consumption resulting from improper setting can be suppressed while plasma is generated with reliability, so that the exhaust-gas cleaning function is exercised sufficiently.
Alternatively, either one electrode out of the discharge electrode and ground electrode is formed in a linear shape, and the other electrode is formed in a cylindrical shape and arranged with the one electrode at the center. The dielectric is composed of a plurality of cylindrical dielectric layers arranged between the electrodes, concentrically around the one electrode out of the electrodes. The plurality of independent cavities are composed of a plurality of independent, cylindrical cavity layers formed between the plurality of dielectric layers. Then, the expression
Vxe2x89xa7Eaxc3x97{xcex5amxc3x97(ra2mxe2x88x92ra1m)/ln(ra2m/ra1m)}xc3x97Rxc3x97xcexa3{ln(ra2n/ra1n)/xcex5an+ln(rs2k/rs1k)/xcex5sk}
is satisfied for an m-th cavity layer, where V is an impressed voltage required for a discharge between the discharge electrode and the ground electrode, Ea is an electric field required for ionization (decomposition of exhaust gas) acting on each of the cavity layers, xcexa3 is a total sum under n=1xe2x88x92N and k=1xe2x88x92K on the assumption that N is the number of cavity layers between the electrodes and K is the number of the dielectric layers, xcex5an is the permittivity of an n-th cavity layer, xcex5sk is the permittivity of a k-th dielectric layer, xcex5am is the permittivity of the m-th cavity layer, ra1n is the inner radius of the n-th cavity layer, ra2n is the outer radius of the n-th cavity layer, rs1k is the inner radius of the k-th dielectric layer, rs2k is the outer radius of the k-th dielectric layer, ra1m is the inner radius of the m-th cavity layer, ra2m is the outer radius of the m-th cavity layer, and R is a relative gas density correlating to temperature and pressure.
Accordingly, the radii ra1n, ra2n, rs1k, and rs2k of the cavity layers and the dielectric layers, the impressed voltage V across the electrodes, and others are put into an ideal relationship.
Therefore, as has been described, detriments such as unnecessary power consumption resulting from improper setting can be suppressed while plasma is generated with reliability. Thus the exhaust-gas cleaning function is exercised sufficiently.
Alternatively, either one electrode out of the discharge electrode and ground electrode is formed in a linear shape, and the other electrode is formed in a cylindrical shape and arranged with the one electrode at the center. The dielectric is composed of a plurality of cylindrical dielectric layers arranged between the electrodes, concentrically around the one electrode out of the electrodes. The plurality of independent cavities are composed of a plurality of independent, cylindrical cavity layers formed between the plurality of dielectric layers. Then, field adjusting portions for occupying a circumferential proportion are arranged in the cavity layers so that inner cavity layers are higher than outer cavity layers in permittivity.
Such an arrangement of field adjusting portions in the individual cavities makes inner cavities higher than outer cavities in permittivity, whereby the cavities increase in capacitance and decrease in electric field toward the inner side.
This makes it possible to suppress radial variations in electric field resulting from the structure of electrode arrangement, thereby allowing uniform plasma density for the sake of higher exhaust-gas cleaning capability.
Alternatively, either one electrode out of the discharge electrode and ground electrode is formed in a linear shape, and the other electrode is formed in a cylindrical shape and arranged with the one electrode at the center. The dielectric is composed of a plurality of cylindrical dielectric layers arranged between the electrodes, concentrically around the one electrode out of the electrodes. The plurality of independent cavities are composed of a plurality of independent, cylindrical cavity layers formed between the plurality of dielectric layers. Here, the radial thicknesses of the cavity layers are smaller on the outer side than on the inner side.
When outer cavities are thus made smaller in thickness, the outer cavities decrease in the flow rate of exhaust gas and then increase in the residence time of exhaust gas, the exhaust gas being subjected to the plasma""s cleaning function for longer periods.
This can suppress radial differences in the cleaning function, thereby realizing yet higher cleaning capability.
Alternatively, either one electrode out of the discharge electrode and ground electrode is formed in a linear shape, and the other electrode is formed in a cylindrical shape and arranged with the one electrode at the center. The dielectric is composed of a plurality of cylindrical dielectric layers arranged between the electrodes, concentrically around the one electrode out of the electrodes. The plurality of independent cavities are composed of a plurality of independent, cylindrical cavity layers formed between the plurality of dielectric layers. Then, protruded portions extending into the cavity layers are arranged on the surfaces of the dielectric layers so that their densities of arrangement are higher on outer dielectric layers than on inner dielectric layers.
When the protruded portions for functioning as portions of nonuniform fields are thus arranged on the surfaces of the dielectrics with their densities of arrangement higher on the outer side, where discharge starting voltage tends to rise with an increase in the radius of curvature, than on the inner side, the starting voltage is uniformized in radial directions.
This allows the voltage applied across the electrodes to act on each cavity uniformly, thereby realizing yet higher exhaust-gas cleaning capability.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific example, while indicating preferred embodiment of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.